Biology subjects

Kikkert, S.

Publications and source records attributed to Kikkert, S..

6 recordsLinked to original sources

bioRxiv2021

Finger representations in primary somatosensory cortex are modulated during vibrotactile working memory

It is well-established that several cortical areas represent vibrotactile stimuli in somatotopic maps. However, whether such somatotopic representations remain active during the delay period of working memory (WM) tasks, i.e. in the absence of any tactile stimulation, is unknown. In our experiment, participants had to compare two tactile stimuli with different vibration frequencies that were separated by a delay period (memory condition) or they were exposed to identical stimuli but did not have to solve a WM task (no memory condition). Importantly, both vibrotactile stimuli were either applied to the right index or little finger. Analyzing the delay period, we identified a well-known fronto-parietal network of brain regions involved in WM but we did not find WM specific activity in S1. However, using multi-voxel pattern analysis (MVPA) and representational similarity analysis (RSA), we found that S1 finger representations were more dissimilar during the delay period of the WM condition than during the control condition. These results indicate that WM processes modulate the representational geometry of S1 suggesting that some aspects of the tactile WM content are represented in a somatotopic fashion. HIGHLIGHTSO_LIMultivariate approaches were used to identify finger specific representational changes during vibrotactile frequency discrimination. C_LIO_LIVibrotactile working memory modulates somatotopic finger representations in contralateral S1 during the delay period, i.e. in the absence of any tactile stimuli C_LI

neuroscience↗

bioRxiv2021

Hand and face somatotopy shown using MRI-safe vibrotactile stimulation with a novel Soft Pneumatic Actuator (SPA)-Skin interface

The exact somatotopy of the human facial representation in the primary somatosensory cortex (S1) remains debated. One reason that progress has been hampered is due to the methodological challenge of how to apply automated vibrotactile stimuli to face areas in a manner that is: 1) reliable despite differences in the curvatures of face locations; and 2) MR-compatible and free of MR-interference artefacts when applied in the MR head-coil. Here we overcome this challenge by using soft pneumatic actuator (SPA) technology. SPAs are made of a soft silicon material and can be in- or deflated by means of airflow, have a small diameter, and are flexible in structure, enabling good skin contact even on curved body surfaces (as on the face). To validate our approach, we first mapped the well-characterised S1 finger layout using this novel device and confirmed that tactile stimulation of the fingers elicited characteristic somatotopic finger activations in S1. We then used the device to automatically and systematically deliver somatosensory stimulation to different face locations. We found that the forehead representation was least distant from the representation of the hand. Within the face representation, we found that the lip representation is most distant from the forehead representation, with the chin represented in between. Together, our results demonstrate that this novel MR compatible device produces robust and clear somatotopic representational patterns using vibrotactile stimulation through SPA-technology.

neuroscience↗

bioRxiv2021

Finger representation in the cortex of the congenitally blind

Hand representation in the primary somatosensory cortex (S1) is thought to be shaped by experience. Individuals with congenital blindness rely on their sense of touch for completing daily tasks that in sighted people would be informed by vision, and possess superior tactile acuity. It has therefore been proposed that their S1 hand representation should differ from that of sighted individuals. Alternatively, it has been proposed that the improved tactile acuity in blind individuals is due to cross-modal plasticity, when regions in the occipital and temporal cortex are typically used for processing vision become activated by touch. We probed finger representation using psychophysics and 7T fMRI (1 mm3 resolution) in three individuals with bilateral anophthalmia, a rare condition in which both eyes fail to develop, as well as sighted controls. Despite anophthalmic individuals increased reliance on touch and superior tactile acuity, we found no evidence that they had more pronounced hand representation in S1. This is in line with recent research highlighting the stability of early sensory cortex, despite altered sensorimotor experience in adulthood. Unlike sighted controls, anophthalmic individuals activated the left human middle temporal complex (hMT+) during finger movement. This area did not express any hallmark of typical sensorimotor organisation, suggesting this and previously reported activity does not indicate low-level sensorimotor hand representation. However, left hMT+ contained some single finger information, beyond that found in sighted controls. This latter finding suggests that when the developmentally flexible area hMT+ is unaffected by retinal input, it can acquire novel cross-modal processes, which are potentially unrelated to the areas function in sighted people. As such, our findings highlight the opportunity for other organising principles, beyond domain specific plasticity, in shaping cross-modal reorganisation.

neuroscience↗

bioRxiv2021

Mental individuation of imagined finger movements can be achieved using TMS-based neurofeedback

Neurofeedback (NF) in combination with motor imagery (MI) can be used for training individuals to volitionally modulate sensorimotor activity without producing overt movements. However, until now, NF methods were of limited utility for mentally training specific hand and finger actions. Here we employed a novel transcranial magnetic stimulation (TMS) based protocol to probe and detect MI-induced motor activity patterns in the primary motor cortex (M1) with the aim to reinforce selective facilitation of single finger representations. We showed that TMS-NF training but not MI training with uninformative feedback enabled participants to selectively upregulate corticomotor excitability of one finger, while simultaneously downregulating excitability of other finger representations within the same hand. Successful finger individuation during MI was accompanied by strong desynchronisation of sensorimotor brain rhythms, particularly in the beta band, as measured by electroencephalography. Additionally, informative TMS-NF promoted more dissociable EEG activation patterns underlying single finger MI, when compared to MI of the control group where no such feedback was provided. Our findings suggest that selective TMS-NF is a new approach for acquiring the ability of finger individuation even if no overt movements are performed. This might offer new treatment modality for rehabilitation after stroke or spinal cord injury.

neuroscience↗

bioRxiv2021

Finger somatotopy is preserved after tetraplegia but deteriorates over time

Following spinal cord injury (SCI), the motor output flow to the limb(s) and sensory input to the brain is largely lost. While attempted movements with the paralysed and sensory deprived body part can still evoke signals in the sensorimotor system, this task-related "net" brain activity of SCI patients differs substantially from healthy controls. Such reorganised and/or altered activity is thought to reflect abnormal processing. It is however possible that this altered net sensorimotor activity in SCI patients conceals preserved somatotopically-specific representations of the paralysed and sensory deprived body parts that could be exploited in a functionally meaningful manner (e.g. via neuroprosthetics). In this cross-sectional study, we investigated whether a functional connection between the periphery and the brain is necessary to maintain somatosensory representations. We used functional MRI and an (attempted) finger movement task to characterise the somatotopic hand layout in the primary somatosensory cortex and structural MRI to assess spared spinal tissue bridges. We tested 14 tetraplegic SCI patients (mean age {+/-} s.e.m.=55 {+/-} 3.6; 1 female) who differed in terms of lesion completeness, retained sensorimotor functioning, and time since injury, as well as 18 healthy control participants (mean age {+/-} s.e.m.=56 {+/-} 3.6 years; 1 female). Our results revealed somatotopically organised representations of patients hands in which neighbouring clusters showed selectivity for neighbouring fingers in contralateral S1, qualitatively similar to those observed in healthy controls. To quantify whether these representations were normal in tetraplegic SCI patients we correlated each participants intricate representational distance pattern across all fingers (revealed using representational similarity analysis) with a canonical inter-finger distance pattern obtained from an independent sample. The resulting hand representation typicality scores were not significantly different between patients and controls. This was even true when considering two individual patients with no sensory hand functioning, no hand motor functioning, and no spared spinal tissue bridges. However, a correlational analysis revealed that over years since SCI the hand representation typicality in primary somatosensory cortex deteriorates. We show that somatosensory representations can be maintained for several years following SCI even in the absence of perhiperhal inputs. Such preserved cortical hand representations could therefore be exploited in a functionally meaningful way by rehabilitation approaches that attempt to establish new functional connections between the hand and the brain after an SCI (e.g. through neuroprosthetics). However, time since injury may critically influence the somatotopic representations of SCI patients and might thereby impact the success of such rehabilitation approaches.

neuroscience↗

bioRxiv2020

Malleability of the cortical hand map following a finger nerve block

Individual fingers in the primary somatosensory cortex (S1) are known to be represented separately and adjacently, forming a cortical hand map. Electrophysiological studies in monkeys show that finger amputation triggers increased selectivity to the neighbouring fingers within the deprived S1, causing local reorganisation. Neuroimaging research in humans, however, shows persistent S1 finger representation of the missing hand, even decades after amputation. We aimed to resolve these apparently contrasting evidence by examining finger representation in humans following pharmacological amputation using single-finger nerve block and 7T neuroimaging. We hypothesised that beneath the apparent selectivity of individual fingers in the hand map, peripheral and central processing is distributed across fingers. If each finger contributes to the cortical representation of the others, then localised input loss will weaken finger representation across the hand map. For the same reason, the non-blocked fingers will stabilise the blocked fingers representation, resulting in persistent representation of the blocked finger. Using univariate selectivity profiling, we replicated the electrophysiological findings of local S1 reorganisation. However, more comprehensive analyses confirmed that local blocking reduced representation of all fingers across the entire hand area. Importantly, multivariate analysis demonstrated that despite input loss, representation of the blocked finger remained persistent and distinct from the unblocked fingers. Computational modelling suggested that the observed findings are driven by distributed processing underlying the topographic map, combined with homeostatic mechanisms. Our findings suggest that the long-standing depiction of the somatosensory hand map is misleading. As such, accounts for map reorganisation, e.g. following amputation, need to be reconsidered.

neuroscience↗